1. Field of the Invention
The present invention relates to an iodine-sulfur cycle for nuclear hydrogen production, which can decrease the corrosivity in operational environments and improve thermochemical efficiency by optimizing process conditions.
2. Description of the Related Art
Recently, Kyoto Protocol for preventing global warming has come into effect and oil prices are unpredictably changing as the fossil fuels are getting exhausted, and thus hydrogen energy has been considered as an alternative energy source in order to decrease the emission of carbon dioxide and secure an economical energy source. Accordingly, many attempts to develop processes of economical hydrogen production using high-temperature nuclear energy have been made world-wide. Among the processes of producing hydrogen, an iodine-sulfur cycle is known as the most promising thermo-chemical cycle.
The iodine-sulfur cycle includes a Bunsen reaction process, a sulfuric acid (H2SO4) decomposition process, and a hydrogen iodide (HI) decomposition process. The basic principle of the iodine-sulfur cycle was first presented by General Atomics Co., Ltd. in U.S.A in the early 1980's (refer to FIG. 1).
According to a typical iodine-sulfur cycle, in a Bunsen reaction process, sulfur dioxide (SO2) and iodine (I2) dissolve in water (H2O) and then react with each other at a relatively low temperature of about 120° C. to decompose water molecules and thus to produce sulfuric acid (H2SO4) and hydrogen iodide (HI) in the form of a mixture. In this case, the change in Gibbs free energy (G) between before and after the reaction must be negative in order to easily conduct the water decomposition reaction, and, for this purpose, a large amount of excess water must be added to the reactants. Subsequently, the mixture discharged from a Bunsen reactor is separated into a relatively-light sulfuric acid solution and a relatively-heavy hydrogen iodide solution through a spontaneous liquid-liquid phase separation process, and, for this purpose, a large amount of excess iodine is required.
The sulfuric acid solution and hydrogen iodide solution produced through the Bunsen reaction process are transferred to a sulfuric acid decomposition process and a hydrogen iodide decomposition process, respectively (refer to FIG. 2). In this case, the sulfuric acid solution includes a large amount of water, an extremely small amount of iodine and an extremely small amount of hydrogen iodide, and the hydrogen iodide solution includes a large amount of water, a large amount of iodine and an extremely small amount of sulfuric acid.
In the sulfuric acid decomposition process, before the sulfuric acid decomposition is conducted, low-concentration sulfuric acid is concentrated in order to decrease the consumption of thermal energy used in a process of heating sulfuric acid at high temperature. The concentrated sulfuric acid solution is heated to about 500° C. and thus easily and rapidly decomposed into water (H2O) and sulfur trioxide (SO3). The produced sulfur trioxide (SO3) is heated to about 850° C. and thus decomposed into oxygen (O2) and sulfur dioxide (SO2). Among the decomposition products, oxygen is separated as one of final products, and water and sulfur dioxide recovered from the sulfuric acid concentration and decomposition processes are cooled and then re-circulated to the Bunsen reaction process (refer to FIG. 2).
The high-temperature thermal energy required in the sulfuric acid concentration and decomposition processes is supplied from a high-temperature nuclear reactor. The sulfuric acid solution used in the sulfuric acid concentration and decomposition processes causes apparatuses used in these processes to be aged because it has high corrosive properties in high-temperature environment.
In the hydrogen iodide decomposition process, there are problems in that the decomposition ratio of hydrogen iodide can be greatly decreased by an extremely small amount of iodine included in the hydrogen iodide solution, and the energy efficiency in the hydrogen iodide decomposition process can be decreased by a large amount of excess water included in the hydrogen iodide solution. For this reason, the hydrogen iodide decomposition process is conducted after minimizing the amount of excess iodine and water included in the hydrogen iodide solution. When the hydrogen iodide solution from which excess iodine and water are removed is heated to about 450° C., the heated hydrogen iodide molecules are thermally decomposed into hydrogen (H2) and iodine (I2). The hydrogen (H2) produced through the heating of the hydrogen iodide is separated as another final product, and water and iodine recovered from the hydrogen iodide concentration and decomposition processes are re-circulated to the Bunsen reaction process (refer to FIG. 2).
The hydrogen iodide decomposition process is problematic in that the amount of energy consumed in the iodine separation process and the excess water circulation process is larger than the amount of energy consumed in the decomposition of hydrogen iodide molecules, and in that although the hydrogen iodide solution has very high corrosivity, its detailed thermo-chemical properties are not well known yet.
As described above, problems to be solved in the use of the iodine-sulfur cycle for nuclear hydrogen production are as follows. First, the corrosivity of sulfuric acid and hydrogen iodide must be overcome. The corrosivity of sulfuric acid and hydrogen iodide increases with the increase of process temperature and pressure. Therefore, in order to overcome this corrosivity of sulfuric acid and hydrogen iodide, there is a method of fabricating apparatuses using materials having excellent corrosion resistance, but this method is also problematic in that economical efficiency is decreased because the apparatuses made of these corrosion resistance materials are expensive.
Second, a large amount of excess water and iodine supplied in the Bunsen reaction process must be recovered and recirculated in subsequent processes. However, even in this case, there is a problem in that a large amount of thermal energy is consumed during the recovery and recirculation of excess water and iodine, thus decreasing the energy efficiency of the entire iodine-sulfur cycle.
In order to solve the above problems, General Atomics Co., Ltd. in U.S.A. proposed a method of increasing the concentration of a hydrogen iodide solution higher than an azeotropic point, in which water is removed from the hydrogen iodide solution using phosphoric acid (H3PO4) as an intermediate substance and then the phosphoric acid is additionally recovered. However, this method is also disadvantageous in that a process of recovering phosphoric acid is additionally required, and thus energy consumption is increased and the entire process is complicated.
Further, Aachen University of Technology in Germany proposed a reactive distillation technology in which a hydrogen iodide solution is simultaneously concentrated and decomposed in a reactive distillation column. However, this reactive distillation technology is also problematic in that high temperature is required in order to decompose hydrogen iodide, and vapor-liquid equilibrium must be maintained throughout the reactive distillation column in order to distill the hydrogen iodide solution for the purpose of concentrating the hydrogen iodide solution, so that high-temperature/high-pressure operation conditions are inevitably required, with the result that the basic problem that the operational environment in the reactive distillation column is not suitable to prevent the corrosion of the reactive distillation column cannot be solved.
Furthermore, Japan Atomic Energy Agency (JAEA), called Japan Atomic Energy Research Institute (JAERI) in the past, has researched an electro-electrodialysis (EED) technology in which a membrane technology and an electrolysis technology are combined in order to concentrate a hydrogen iodide solution. However, this electro-electrodialysis (EED) technology is also problematic in that the basic limitation that a large amount of expensive electrical energy is required must be overcome, and in that the technical difficulty that the size of the membrane, which is structurally weak, must be increased from that of a small laboratory scale to that of a large hydrogen production plant scale must also be solved, thereby commercially using this electro-electrodialysis (EED) technology.
Therefore, the present inventors found that a process for minimizing the amount of excess water and iodine supplied to a Bunsen reaction process can minimize the amount of thermal energy consumed in the recovery and recirculation process thereof, that sulfuric acid having stronger hydrophilicity than hydrogen iodide absorbs excess water in large quantities in a spontaneous liquid-liquid phase separation process, so that, after the a spontaneous liquid-liquid phase separation process, the concentration of hydrogen iodide in a hydrogen iodide solution exceeds a concentration at the azeotropic point without conducting an additional concentration process, with the result that highly-concentrated hydrogen iodide gas can be obtained only through a flashing process, thereby decreasing energy consumption and simplifying the process and thus improving economical efficiency, and that process temperature and pressure can be decreased, thus greatly deceasing the corrosivity in operational environments.